I've never used them. I do have a fiberglass tub with original seat brackets, but mine's an 84 CJ7. How thin is thin on the fiberglass? Anytime you're bolting anything to fiberglass that needs any structure, use a large backing plate or Big fender washers to distribute the load over as larger area. I used 2" on a lot of my bolted items. Some I used aluminum or stainless backing plates made from flat plate.
I'm not sure what tomorrow will bring, but it can't be any worse than what I've already been through.